The present invention relates to a movable coil-type actuator for moving a function member such as a magnetic head in a magnetic disk drive and an optical head in a CD or LD drive or a magneto-optical disk drive, along a circular or straight path, and a recording apparatus comprising such a movable coil-type actuator.
One of conventional movable coil-type actuators is shown in FIGS. 9 to 11. FIG. 9 is a partly cross-sectional front elevation, FIG. 10 is a cross-sectional view taken along the line C--C in FIG. 9, and FIG. 11 is an exploded perspective view. A base yoke element 1, a center yoke element 2 and side yoke elements 3, 3 are rectangular plates made of a ferromagnetic material such as soft iron and are joined by bolts or other fixing means (not shown) to make up an E-shaped yoke.
A short ring 4 is a hollow, rectangular-tubular member made of an electrically conductive material such as copper, which encloses the center yoke element 2. Permanent magnets 5, 5 each in the form of a rectangular plate magnetized in its thickness direction are fixed to inner surfaces of the side yoke elements 3, 3, respectively, with the same magnetic pole opposing the center yoke 2. An arm 7 supports a hollow rectangular-cylindrical coil (movable coil) 6 at one end and a function member such as a magnetic head (not shown) at the other end. The arm 7 is pivotally or swingably supported by a shaft 9 to locate the movable coil 6 in a magnetic gap 8 between the permanent magnets 5, 5 so that the arm 7 moves in the magnetic gap 8 along surfaces of the permanent magnets 5, 5. A counter yoke element 10 is a flat, rectangular member made of the same material as that of the elements 1, 2, 3 constituting the E-shaped yoke, and secured to the open end of the E-shaped yoke by screws or other fixing means (not shown).
When a signal current is supplied to the movable coil 6, a magnetic force acts on the movable coil 6 according to the Fleming's left hand rule and makes the arm 7 pivot or swing about the shaft 9. As a result, a function member such as a magnetic head supported at the other end of the arm 7 is brought to a desired recording track on a magnetic disk. The swinging direction of the arm 7 is changed by inverting the direction of a signal current applied to the movable coil 6.
The actuator shown in FIGS. 9 and 10 has been fabricated by forming individual yoke elements 1 to 3, joining them together into an E-shaped yoke as shown in FIG. 11 by appropriate fixing means, and then bonding the permanent magnets 5, 5 to inner surfaces of the side yoke elements 3, 3 by an adhesive. Such an adhesive, however, must be set typically by heating, which needs additional time and steps and hence increases a manufacturing cost of the actuator. In addition, gas generated from the adhesive during the heating process invites an environmental problem.
Another conventional movable coil-type actuator is shown in FIGS. 12(a), 12(b), 13 and 14. FIG. 12(a) is a partly broken plan view, FIG. 12(b) is an elevational view taken from the line D--D in FIG. 12(a), FIG. 13 is a perspective view, and FIG. 14 is an exploded perspective view. Plate-shaped yoke elements 11a, 11b are made of a ferromagnetic material such as soft iron and opposing each other via supports 21, 21 which connect their end portions. A trapezoidal plate-shaped permanent magnet 31 (or a combination of two magnets) magnetized in the thickness direction thereof is fixed to an upper surface of a lower yoke element 11b such that N and S magnetic poles appear in a magnet surface exposed to a magnetic gap 41 defined between the upper yoke element 11a and the permanent magnet 31.
An arm 51 supports a flat movable coil 61 at one end and a function member such as a magnetic head (not shown) at the other end. The arm 51 is pivotally or swingably supported by a shaft 71 to locate and move the movable coil 61 in the magnetic gap 41 defined by the upper yoke element 11a and the permanent magnet 31.
The lower yoke element 11b is provided with stoppers 81, 81 projecting therefrom for engaging the arm 51 to prevent its undesired movement when the arm 51 should rest. The lower yoke element 11b is also provided with stopper pins 81a, 81a projecting therefrom near opposite ends of the permanent magnet 31 to regulate the swing span of the flat movable coil 61 (omitted in FIGS. 12(b) and 13). Positioning pins 91 projecting from the lower yoke element 11b are brought into contact with a periphery of the permanent magnet 31 to hold it in place.
When a signal current is supplied to the flat movable coil 61, the magnetic head supported at the other end of the arm 51 is brought to a desired recording track on a magnetic disk in the same manner as in the actuator shown in FIGS. 9 to 11.
The actuator shown in FIGS. 12(a) through 14 is fabricated by forming individual elements, fixing the positioning pins 91 to the lower yoke element 11b at proper positions as shown in FIG. 14, and then bonding the permanent magnet 31 to the lower yoke element 11b by an adhesive. This actuator, therefore, suffers from the same adhesive-related problem as mentioned with reference to the actuator shown in FIGS. 9 to 11.
Still referring to FIGS. 12(a) to 14, after the permanent magnet 31 is bonded to the lower yoke element 11b, this conventional actuator needs subsequent steps of fixing the supports 21, 21 and the stoppers 81, 81 to the lower yoke element 11b, fixing the upper yoke element 11a to the supports 21, 21 by screws 21a, 21a. Since the fixing of these elements and members requires screws or caulking, the assembling process needs much manual labor, taking much time and many steps and resulting in a high production cost.
With respect to the actuator shown in FIGS. 12(a) to 14, there have been made several proposals to maximize the uniformity of the magnetic density distribution in the magnetic gap 41. For instance, a notch may be provided in a periphery of the permanent magnet 31 at a center position thereof (see, for example, Japanese Utility Model Laid-Open No. 3-86778). However, when the permanent magnet 31 is provided with a notch, a complicated casting die is needed, and it is likely to cause a defective molding.
Alternatively, the thickness of a central portion of the permanent magnet 31 may be changed to increase the dimension of the magnetic gap 41 (see, for example, Japanese Patent Laid-Open No. 4-114373). However, when the thickness of the permanent magnet is changed, mechanical working of the permanent magnet 31 originally having a uniform thickness is needed, increasing a production cost.
Further, one of the yoke elements 11a, 11b opposing the permanent magnet 31 may be made to have such a configuration that the dimension of the magnetic gap 41 diminishes from the center toward the end of the permanent magnet 31 (see, for example, Japanese Patent Laid-Open No. 64-77455). However, when the yoke element is provided with a special shape, an additional working cost is needed.
Considering an increasingly higher demand of making movable coil-type actuators thinner and less expensive, the actuators shown in FIGS. 9 to 11 and FIGS. 12(a) to 14 are unsatisfactory because they need many parts and a high assembling cost.
With respect to a magnetic disk drive, FIGS. 15-17 show a conventional one. FIG. 15 is a partly cut-away plan view, FIG. 16 is a partial cross-sectional view, and FIG. 17 is an exploded perspective view. The magnetic disk drive comprises a rectangular casing 210 and a cover 222 both made of a non-magnetic material such as an aluminum alloy. Disposed in the casing 210 is a driving motor (not shown) having a spindle 223 to which a plurality of magnetic disks 224 are fixed with a certain interval. A swingable arm 205 has a flat movable coil 206 at one end and a magnetic head 226 at the other end, and is swingable about a shaft 207 fixed to the casing 210. The flat movable coil 206 is positioned in a magnetic circuit formed by a permanent magnet 31 and a pair of yokes 211a, 211b, and the magnetic head 226 is positioned on a magnetic track of each magnetic disk 224.
A magnetic circuit-forming means comprises an upper yoke element 211a and a lower yoke element 211b both made of a ferromagnetic material such as soft iron and opposing each other via supports 221, 221 which connect their end portions. A permanent magnet 31 in a substantially trapezoidal flat shape and magnetized in the thickness direction thereof is fixed to an upper surface of a lower yoke element 211b such that N and S magnetic poles appear in a magnet surface exposed to a magnetic gap 204 defined between the upper yoke element 211a and the permanent magnet 31. The flat movable coil 206 is movable back and forth in the magnetic gap 204 and its swinging direction can be changed by inverting the direction of a signal current applied to the flat movable coil 206.
When a signal current is supplied to the flat movable coil 206, a magnetic force acts on the movable coil 206 according to the Fleming's left hand rule and makes the arm 205 pivot or swing about the shaft 207. As a result, the magnetic head 226 supported at the other end of the arm 205 is brought to a desired recording track on the magnetic disk 224.
FIG. 17 is an exploded perspective view of the magnetic circuit-forming means (movable coil-type actuator) contained in the above magnetic disk drive. The lower yoke element 211b is provided with positioning pins 209 which are brought into contact with a periphery of the permanent magnet 31 for holding it at a proper position.
To form the above magnetic circuit-forming means, individual elements are joined together, and the permanent magnet 31 is positioned by the positioning pins 209 and bonded to an upper surface of the lower yoke element 211b by an adhesive. The use of an adhesive, however, causes the same problems as discussed above.
Since the fixing of the permanent magnet 31 to the lower yoke element 211b with the supports 221, 221 requires screws or caulking, the assembling process needs much manual labor, taking much time and many steps and resulting in a high production cost, failing to meet the recent demand of making the magnetic disk drive thinner and less expensive.